Laser cutting, e.g. of metal plate, is usually performed with the aid of a jet of gas directed onto the cutting area for flushing out the liquified material.
The type of gas used depends on the workpiece material, and must be so selected as to prevent undesired chemical reactions affecting the cutting faces. It is particularly important to prevent the formation of products affecting the metallurgical structure of the cutting faces, and so resulting in hardness or fragility preventing post-process machining or actual use of the part, if no further machining is required.
Oxygen is excellent for cutting ferrous materials (alloys), due to the exothermic reaction produced at high temperatures (over 720.degree. C.). If properly employed, the energy and fluid-thermodynamic effects so produced may result in increased cutting speed for a given power of the laser, improved flushout of the liquefied material, and a better surface finish of the cutting faces.
To understand the chain of events occurring in this process, some knowledge is required of the fluid-thermodynamics of reactive gases, and the complex phenomena produced when such a gas is subjected to intense heat by both the laser beam and the liquified material, and the mass of the gas stream is increased by the presence of the liquid. This results in a gas stream consisting of a mixture of compressible gas and (incompressible) liquid, with all the possibilities this entails. For example, the liquid may be broken down into large or small drops, or even atomized.
Depending on the amount of heat, particularly that radiated by the laser beam and high-temperature solid or fluid surfaces, and the combined thermal and fluid-dynamic effects involved, the drops and atomized liquid particles may be converted into steam, which, though undesirable, is nevertheless inevitable. Moreover, due to their small size and the absence of a cooling mass (the cold faces of the workpiece), the liquid particles within range of the laser beam may also be converted into steam, which, like the existing steam, may be energized or even ionized by the laser beam, thus resulting in several of the luminous phenomena observed during the cutting process.
In dealing with the above phenomena, the reactivity of the gas must also be taken into account. In the case of oxygen, if the temperature of the liquid species exceeds a given threshold value (720.degree. C.), an intensely exothermic reaction is initiated, which further accentuates the intensity and instability of the above phenomena, and is what accounts for the more or less periodic oscillation of the various components of the fluid-thermodynamic field. Another important point to note is that the liquid phase subjected to the laser beam may be induced to oscillate at an entirely different amplitude and frequency as compared with those deposited and/or formed on the cutting face. The liquid phases immersed in the gas stream may alter in limited regions the frequency and amplitude of both the free-flow stream at the start of the cut, and that channelled between the cutting faces. The dynamic action of the various phases so interact as to affect the end result of the process in ways and to an extent varying widely depending on the operating parameters and equipment employed.
For example, on commonly used laser cutting equipment, the laser beam is normally focused in a conical nozzle, through which the oxygen is also supplied, with no separation between it and the laser beam, and directed freely on to the cutting area. Moreover, using a conical nozzle located a given distance from the work surface, the oxygen jet forms a round impact spot about the cut, the diameter of which may be as much as 5 to 10 times the width of the cut, which attempts are made to make as narrow as possible.
The above known method presents numerous drawbacks.
Firstly, oxygen consumption is high, due to a substantial percentage being directed to no purpose on to the area surrounding the cut. In the above example, for instance, in which the diameter of the impact spot is 5 to 10 times the width of the cut, consumption is respectively 8 to 30 times that actually required. Secondly, by virtue of contacting a large portion of the laser beam for a considerable length of time, the oxygen is heated to an extremely high temperature by the time it reaches the workpiece.
The effects of such heating are numerous. Firstly, the coefficient of refraction varies irregularly, thus impairing focusing of the laser beam, which is further affected by the convective motion produced by heating the oxygen.
If the gas is heated long enough for it to reach, firstly, the thermal excitation and then the thermal ionization threshold, this may (even at laser cutting power levels) result in dissipation absorbing the power of the laser beam.
The reduction in power and defocusing of the beam combine to reduce the power and increase the diameter of the focal spot, both of which are unfavourable for obtaining as narrow a cut as possible. Moreover, the instability caused by both convective motion and the flow phenomena required for the gas jet to penetrate inside the cut results in unsteady phenomena which also affect cutting efficiency and the quality of the cutting faces: scoring, tears, undesired metallurgical properties.
Most of these drawbacks are caused by using a nozzle which directs the gas jet freely on to the cutting groove surface.
Attempts to increase flow velocity and so improve flushout of the liquid by increasing the pressure of the jet are pointless and even counterproductive in the case of commonly used conical nozzles. When the pressure of the jet is increased over and above the critical ratio, in fact, this results in instability in the direction and velocity of the jet (wobble and pulsation), which further affect the alteration in flow caused by internal and external aerodynamic factors (boundary layers, atmospheric air mix and drag), thus resulting in substantially uncontrollable situations.
The instability of a freely-directed jet makes it even more difficult for the jet to enter and penetrate inside the cut, thus aggravating the "choking" phenomenon typical of subsonic and supersonic jets. In the conduit portion downstream from the choking section, velocity is significantly reduced. Traditional cutting faces therefore present a longitudinal line (i.e. parallel to the top and bottom surfaces of the workpiece) caused by the choking effect, and, up- and downstream from this, a series of differently sloping score lines indicating a change in flow velocity. In particular, the slope of the score lines is greater downstream from the choking line, thus indicating a reduction in flow velocity.
A significant reduction in flow velocity also results in a variation in scoring frequency, as well as in erosion accompanied by droplets of liquified material, all of which are caused by the uncontrolled exothermic reaction produced by greater penetration of the face by the isotherm (about 720.degree. C.) initiating the reaction.